Many particular methods and systems for controlling motors of various types have been used commercially. Some methods for controlling a motor use an external sensor, such as an optical encoder or a Hall effect sensor to provide position information necessary to commutate or regulate the motor. However, in some applications, using an external sensor is undesirable due to the increased cost. An example in which an external sensor is undesirable includes a low cost printer or a hard disk drive using a step motor or a three phase brushless dc (BLDC) motor, respectively.
To address this external sensor cost concern, some step motors (U.S. Pat. No. 5,227,709) and BLDC motor (U.S. Pat. No. 4,162,435) systems avoid driving one of the motor phases and use an integrated voltage of the back electromotive force (BEMF) of the undriven motor phase to provide position information. For both motor types, such systems yield only a coarse estimate of position information to determine a next commutation point for the motor's stator. Using an undriven motor phase to provide position information leads to the motor having reduced maximum torque, increased torque ripple, and a lesser degree of velocity control, thereby reducing performance and effectively increasing cost to achieve the same torque requirements as compared to the same motor in which all motor phases are driven.
An alternate method to sensing the back EMF of a driven phase for a step motor is described in U.S. Pat. No. 4,658,194. This method takes an estimate of the back EMF by estimating the equation Uemf=iR+L(di/dt)-U. This method requires an estimate of the phase current through an external sense resistor which is not always desirable since power that could be delivered to the motor is now being dissipated in the external sense resistor. In addition, this method requires a considerable amount of circuitry to estimate the back EMF.
An improvement to U.S. Pat. No. 4,162,435 for BLDC motors is made in U.S. Pat. No. 4,743,815 and U.S. Pat. No. 5,422,570. In U.S. Pat. No. 4,743,815 a decode step is introduced which makes use of zero crossing information which is simpler than integrating the back EMF voltage. In U.S. Pat. No. 5,422,570 the third harmonic is extracted from a three phase, eye wound, trapezoidal back EMF motor by summing the terminal voltages. The zero crossing information is then used to control the motor. The methods described in these two patents have limitations requiring a BLDC motor:
U.S. Pat. Nos. 4,743,815 and 5,422,570 use a six state controller methodology in which one phase of the motor remains undriven and only provide 60 degrees of electrical resolution (position information),
U.S. Pat. No. 4,743,815 requires a decode unit to determine zero crossings leading to increased circuitry and cost.
U.S. Pat. No. 5,422,570 requires a summation of the motor terminals to extract a third harmonic component and requires that the amplitude of the third harmonic component be larger than all harmonics except the primary. The method assumes a trapezoidal back EMF motor type (not sinusoidal which has better torque ripple characteristics) with a pole pitch other than 2/3 (a 2/3 pole pitch design does not have a significant 3rd harmonic). Due to the limitation and assumptions of a particular motor type, many motor applications, such as a printer carriage driven by a step motor, may not be addressed by using the teachings of U.S. Pat. No. 5,422,570.
U.S. Pat. No. 4,743,815 produces an output signal that contains spurious switching instants created by switching the motor drivers to the next commutation state. The switching instants are a problem for the controller at higher motor RPM's. U.S. Pat. No. 5,422,570 requires a high pass filter to remove the switching instants, all at further increased cost.
Accordingly, there is a need for an improved method and system of motor control that does not apply stringent conditions to the motor type. Ideally, this new method and system should operate without an undriven phase or external sensor and should increase the amount of position information available to the controller.